Re: [PATCH] btrfs: fix NULL pointer deref during assertion in btrfs_backref_free_node()

在 2026/6/30 00:39, [email protected] 写道:
> From: Filipe Manana <[email protected]>
> 
> In btrfs_backref_free_node() we have the following assertion:
> 
>    ASSERT(node->eb == NULL, "node->eb->start=%llu", node->eb->start);
> 
> and a user reported the following crash:
> 
>    Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI
>    KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
>    CPU: 0 UID: 0 PID: 10422 Comm: syz.0.17 Not tainted 7.1.0-02765-g6b5a2b7d9bc1-dirty #44 PREEMPT(full)
>    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
>    RIP: 0010:btrfs_backref_free_node fs/btrfs/backref.c:3057 [inline]
>    RIP: 0010:btrfs_backref_free_node+0xb9/0x200 fs/btrfs/backref.c:3051
>    Code: 00 fc ff (...)
>    RSP: 0018:ffa0000006b0f3c0 EFLAGS: 00010246
>    RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff840eb78b
>    RDX: 0000000000000000 RSI: ffffffff840eafa5 RDI: ff110000742ab768
>    RBP: ff110000742ab700 R08: 0000000000000000 R09: 0000000000000000
>    R10: ff110000742ab700 R11: 00000000000a81f9 R12: ff11000107a92020
>    R13: ff1100005c182ea8 R14: 0000000000000000 R15: dffffc0000000000
>    FS:  0000555575536500(0000) GS:ff11000183985000(0000) knlGS:0000000000000000
>    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>    CR2: 00007fa3d0e9d580 CR3: 000000002232a000 CR4: 0000000000753ef0
>    PKRU: 00000000
>    Call Trace:
>     <TASK>
>     btrfs_backref_cleanup_node+0x27/0x30 fs/btrfs/backref.c:3133
>     relocate_tree_block fs/btrfs/relocation.c:2604 [inline]
>     relocate_tree_blocks+0x11b0/0x1a20 fs/btrfs/relocation.c:2707
>     relocate_block_group+0x499/0xf30 fs/btrfs/relocation.c:3635
>     do_nonremap_reloc fs/btrfs/relocation.c:5323 [inline]
>     btrfs_relocate_block_group+0x1749/0x5fb0 fs/btrfs/relocation.c:5490
>     btrfs_relocate_chunk+0x12b/0x950 fs/btrfs/volumes.c:3647
>     __btrfs_balance fs/btrfs/volumes.c:4586 [inline]
>     btrfs_balance+0x1c7f/0x55c0 fs/btrfs/volumes.c:4973
>     btrfs_ioctl_balance fs/btrfs/ioctl.c:3474 [inline]
>     btrfs_ioctl+0x38a4/0x5d20 fs/btrfs/ioctl.c:5570
>     vfs_ioctl fs/ioctl.c:51 [inline]
>     __do_sys_ioctl fs/ioctl.c:597 [inline]
>     __se_sys_ioctl fs/ioctl.c:583 [inline]
>     __x64_sys_ioctl+0x18f/0x210 fs/ioctl.c:583
>     do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>     do_syscall_64+0x11f/0x860 arch/x86/entry/syscall_64.c:94
>     entry_SYSCALL_64_after_hwframe+0x77/0x7f
>     RIP: 0033:0x7fb38e3b56dd
>     Code: 02 b8 ff (...)
>     RSP: 002b:00007fff04115788 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
>     RAX: ffffffffffffffda RBX: 00007fb38f6b0020 RCX: 00007fb38e3b56dd
>     RDX: 00002000000003c0 RSI: 00000000c4009420 RDI: 0000000000000004
>     RBP: 00007fb38e451b48 R08: 0000000000000000 R09: 0000000000000000
>     R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
>     R13: 0000000000000000 R14: 00007fb38f6b0020 R15: 00007fb38f6b002c
>     </TASK>
> 
> It seems that this happens on some systems for some reason, when the
> ASSERT() macro calls the inline function verify_assert_printk_format()
> to evaluate the format string and arguments, causing the NULL pointer
> dereference on node->eb.
> 
> So change the assertion to check for a NULL node->eb before dereferencing
> it. Also, while at it, make the assertion more useful by printing the
> owner of the extent buffer as well as its level.
> 
> Reported-by: Yue Sun <[email protected]>
> Link: https://lore.kernel.org/linux-btrfs/[email protected]/
> Fixes: c4e7778580d6 ("btrfs: use verbose assertions in backref.c")
> Signed-off-by: Filipe Manana <[email protected]>

Reviewed-by: Qu Wenruo <[email protected]>

Thanks,
Qu

> ---
>   fs/btrfs/backref.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
> index 23c3eeb58dc1..1be632c742bd 100644
> --- a/fs/btrfs/backref.c
> +++ b/fs/btrfs/backref.c
> @@ -3054,7 +3054,10 @@ void btrfs_backref_free_node(struct btrfs_backref_cache *cache,
>   	if (node) {
>   		ASSERT(list_empty(&node->list));
>   		ASSERT(list_empty(&node->lower));
> -		ASSERT(node->eb == NULL, "node->eb->start=%llu", node->eb->start);
> +		ASSERT(node->eb == NULL, "node->eb->start=%llu level=%d owner=%llu",
> +		       node->eb ? node->eb->start : 0,
> +		       node->eb ? btrfs_header_level(node->eb) : 0,
> +		       node->eb ? btrfs_header_owner(node->eb) : 0);
>   		cache->nr_nodes--;
>   		btrfs_put_root(node->root);
>   		kfree(node);